I'm amazed that this has gone on for so long.
Stand by your guns and simply stop their hosting for overdue fees - you are legally allowed to do so and they have no comeback providing you are correct and they haven't paid, no if's no buts.
As for the domains, you own them so keep them but dont continue their hosting even if they pay their old invoices, as soon as their current hosting expires tell them to go elsewhere, also if they havent taken a recent copy of their site, tough, you dont have to provide it once hosting has expired - again nothing they can do.
So to summarise:
Shut their sites down NOW
Keep their domains, you own them and they have no claim to them.
Dont give them squat unless paid in advance, this includes a copy of "their" site.
Stop letting them push you around, dont waste money on solicitors as they dont have a leg to stand on and can go swivel.